There are several different ways to write Arabic in our Latin alphabet. What follows is an excerpt from the Wikipedia article on Arabic transliteration. This is by far the most widespread method in use and most all Arabs online, when not typing in Arabic characters, use this, so if you ever plan to learn from authentic sources or visit Arabic message boards it will help for you to learn this. It's also the method that I try to adhere to when I'm transliterating. You'll notice that when they add an apostrophe ( ' ) to a number that indicates a dot on top of the Arabic letter. The full article is here.
3 represents the Arabic letter ع .
5 or 7' represent the Arabic letter خ .
6 represents the Arabic letter ط .
6' represents the Arabic letter ظ .
7 represents the Arabic letter ح .
8 represents the Arabic letter ق .
9 represents the Arabic letter ص .
9' represents the Arabic letter ض .
2 is sometimes used to represent the أ when it is in the middle of a word
The numerals 2, 3 and 7 are vastly used in Arabic chatting, because they represent Arabic letters that do not sound like any letter of the roman script. The other numerals can be replaced by roman letters that have a very close pronunciation (for example ظ can be represented by d, ص by s, ق by q) or a combination of roman letters (for example, kh can represent خ).
When numerals are to be avoided, a single quote (') may be used in the place of 2, h in the place of 7 and a single quote (') or double vowels in the place of 3 (for example 3a can become aa).